visit_decref: Assertion `gc->gc.gc_refs != 0' failed.

[Tim Peters]

[alexLIGO]
> I got this error when trying to execute the following python command
> with in a C module:  Py_BuildValue
>
> Do anyone have any idea what this error is about?

Just the comment on the failing assert:

    assert(gc->gc.gc_refs != 0); /* else refcount was too small */

There are more pointers to the object than the object's reference
count says there are.  That's fatally bad.  Probably due to a missing
Py_INCREF, possibly due to an "extra" Py_DECREF.

> And does anyone have any idea how to debug a python script?

You need to debug C code here, not Python code.  It's unlikely that
the problem is in C code that ships with Python; it's far more likely
to be in your C code, or in C code from a third-party extension
module.